1. Field of the Invention
The present invention relates to a continuously variable transmission, and more particularly, to a V belt continuously variable transmission for an automobile.
2. Description of the Related Art
In general, various types of continuously variable transmissions have been proposed and put into practice. A general continuously variable transmission includes a driving pulley, a driven pulley and an endless V belt running across these pulleys, in such a way that the transmission ratio can be continuously variable by changing the running diameters of the belt around the driving pulley and the driven pulley radially in opposite directions. Thus, the continuously variable transmission can implement smooth driving without shift shock.
Japanese Unexamined Patent Application Publication No. 5-280613 (1993) proposes a continuously variable transmission including actuators for changing the running diameters of a belt around a driving pulley and a driven pulley, respectively, a transmission ratio changing mechanism for interlocking the actuators such that the running diameters of the belt around those pulleys are changed radially in opposite directions to each other so as to make the transmission ratio variable, and a tension generating mechanism for pressing the slack side of the belt running across the pulleys so as to have the belt tension larger than that naturally generated depending on the transmission ratio. This tension generating mechanism is formed by an arm swingably supported on a transmission case, a tension roller rotatably mounted on one end of the arm and a spring urging the arm so that the tension roller presses the outer surface of the slack side of the belt.
However, in the tension generating mechanism having the aforementioned structure, where the tension roller is rotatably mounted on one end of the arm and the arm is urged by the spring for pressing the tension roller against the belt, such a problem may occur that the tension roller disadvantageously interferes with the driving pulley or the driven pulley. This is because the tension roller moves along the rotation locus of the end of the arm. Even if the rotation axis or the length of the arm is set such that the tension roller interferes with neither pulley at the lowest transmission ratio or the lowest vehicle speed ratio, the tension roller may interfere with either pulley at the highest transmission ratio or the highest vehicle speed ratio. Therefore, it is difficult to set the rotation axis or the length of the arm such that the tension roller does not interfere with the pulleys at any transmission ratio. If the distance between the axes of the driving pulley and the driven pulley were increased, the tension roller could be arranged so as not to interfere with either pulley at any transmission ratio. This case, however, would cause a problem in that the size of the transmission body itself is disadvantageously increased.
Even if the tension roller can be arranged so as not to interfere with either pulley in an initial stage, the position of the tension roller may be varied when the pulleys are worn down or the belt is elongated due to endurance driving test or the like. As a result, the tension roller may come to disadvantageously interfere with the pulleys.
Accordingly, an object of the present invention is to provide a continuously variable transmission that is capable of reliably preventing interference between a tension roller and a pulley at any transmission ratio while solving the aforementioned problems.